In the drilling of an oil or a gas well it is advantageous to obtain as much information as possible about the nature of the formations, or beds. To this end, core samples of the formation are obtained. Also, well logging devices are placed into the resulting borehole to measure various characteristic properties of the surrounding formation.
The generation and recording of borehole acoustic waves is a key measurement employed in oil well wireline logging. The traditional parameter to be estimated from the waveforms has been the travel time differences. Other techniques are also known, for instance, semblance processing of borehole acoustic array data, Kimball and Marzetta, Geophysics, Vol. 49, No. 3, March, 1984 at pages 274-281.
Acoustic waveforms, in addition to methods based on conventional logs, have been used to detect and characterize fractures intersecting and surrounding the borehole. These log-based methods include differential response to porosity measurements, magnitude of apparent power law exponent relating to formation porosity, and various resistivity log curves resulting from different responses to fracture conductivity. The acoustic methods, whether seismic, sonic, or ultrasonic, are usually based on the scattering and absorption of acoustic energy associated.with the various wave components contained in the full waveforms as described in the SPE paper by the inventor noted, infra. For instance, recent borehole studies have suggested that the reduction in Stoneley or tube wave energy can be used as a reliable indicator of fractures open to fluid flow.
However, each method is subject to certain limitations in its own application. Various components in the waveform resulting from reflection, refraction and mode conversion are often, if not always difficult to separate. To make matters worse, these interference patterns can also be generated by bed boundaries and borehole irregularities, as well as by the fractures which are sought to be detected and characterized.
Thus, it is readily apparent that there has been an ongoing effor made in this art to derive meaningful information about formations from logging instruments. Improvements lie in the analysis of the signals, usually from conventional wireline logging equipment. The advent of the general purpose digital computer has been an indispensible tool to the engineer seeking to derive ever more meaningful interpretations of data. Incorporation by reference is made herein to U.S. Pat. Nos. 4,314,339, 4,543,648 (MSP) and 4,562,556, (DPD) which are representative of the way in which a compute may be applied to a wireline logging system to develop meaningful data from wireline data (signals).